Interpreting the operation of parameters controlling solute transport is challenging in shales and shale-derived soils because of complex chemical and physical heterogeneity. Quantifying solute transport processes in the weathered shale and soils of the Shale Hills Critical Zone Observatory (SH-CZO) is important in interpreting the residence times of ions in the groundwater system, and consequently weathering rates and age of groundwater. In three 10-cm diameter undisturbed soil columns from the SH-CZO (spanning the intervals from 0-15, 20-35, and 40-50 cm) as well as consolidated shale cores, we are evaluating physical heterogeneity, as well as solute transport parameters such as (1) mobile/immobile porosity, (2) mass-transfer rate between domains, and (3) the behavior of exchangeable ions within the system as material properties shift from soil to consolidated rock. Soil samples change with respect to color and biogenic fabric or structure with depth, and the frequency of shale bedrock fragments within in the soil increases with depth. Constant flow experiments in the soil provide a hydraulic conductivity of 10-6 m/s at 0-15 cm depth. Field-scale hydraulic conductivities from slug and pumping tests of the fractured shale at depth are approximately the same: 10-5 to 10-6 m/s. A hydraulic test in a triaxial compression chamber places the hydraulic conductivity of the shale matrix, however, at less than 10-15 m/s, indicating that fractures control permeable pathways within the shale bedrock. Taken collectively, such observations indicate high physical heterogeneity within the subsurface material that could be controlling flow and transport behavior. Concentration histories from a conservative sodium bromide tracer test carried out on the uppermost soil column exhibit long tailing behavior, indicative of non-equilibrium transport. Numerical modeling with Comsol indicates that a mobile porosity of 0.3 and an immobile porosity of 0.35 coupled by a mass-transfer rate of 1 hr-1 can explain the concentration breakthrough history in this soil. These porosities fit previously published data stating that the total porosity of the soil is greater than 0.6. The high mass-transfer rate (indicative of a diffusion length scale of ~3 mm) implies that the mobile and immobile domains are well connected at the column scale. We are exploring how the relative sizes of the mobile/immobile reservoirs change through the soil profile and into the shale , but expect that without incorporating the capacity of the immobile domain to store solutes in the SH-CZO, we could underestimate solute transport times, physical and chemical weathering rates, and hydrologic cycling in the watershed.
Kuntz, B., Singha, K. (2009): Solute transport in shale and shale-derived soils at the Shale Hills CZO. AGU Annual Fall Conference Proceedings.
This Paper/Book acknowledges NSF CZO grant support.